Power and thermal management are becoming more challenging than ever before in all segments of computer-based systems. While in the server domain it is the cost of electricity that drives the need for low power systems, in mobile systems battery life and thermal limitations make these issues relevant. Managing a computer-based system for maximum performance at minimum power consumption may be accomplished by reducing power to all or part of the computing system when inactive or otherwise not needed.
One power management standard for computers is the Advanced Configuration and Power Interface (ACPI) standard, e.g., Rev. 3.0b, published Oct. 10, 2006, which defines an interface that allows the operating system (OS) to control hardware elements. Many modern operating systems use the ACPI standard to perform power and thermal management for computing systems. An ACPI implementation allows a core to be in different power-saving states (also termed low power or idle states) generally referred to as so-called C1 to Cn states.
When the core is active, it runs at a so-called C0 state, but when the core is idle, the OS tries to maintain a balance between the amount of power it can save and the overhead of entering and exiting to/from a given state. Thus, C1 represents the low power state that has the least power savings but can be switched on and off almost immediately (thus referred to as a “shallow low power” or “shallow idle” state), while deep low power states (e.g., C3, C6 or C7) represent a power state where the static power consumption may be negligible, depending on silicon implementation, but the time to enter into this state and respond to activity (i.e., back to active C0) is relatively long. Note that different processors may include differing numbers of core C-states, each mapping to one ACPI C-state. That is, multiple core C-states can map to the same ACPI C-state.
Current OS C-state policy may not provide the most efficient performance results because current OS C-state policy may not consider activities of other cores in the same package. In particular, current OS C-state policy may fail to take advantage of efficiencies that could be gained by more closely tracking and managing the power states of various threads running on different cores of the same package. That is, one hardware thread of a core may be in a deep low power state while another hardware thread of the core may be active. According to current OS C-state policy, a core cannot enter a deep low power state unless all threads on the core are in a deep low power state. If multiple cores experience this condition, then none of the cores can go into a deep low power state (even if multiple hardware threads are inactive).